In the early years of aluminum metal it was believed that aluminum did not require painting. Current technology, quite to the contrary, strongly recommends painting of aluminum surfaces together with rather elaborate steps for pretreating the metal, i.e. alkaline washings, degreasing, deoxidizing, oxidizing, conversion coatings, inhibitive rinsings and the like prior to application of the organic polymer or resin paint. Aluminum and its alloys require various preparation and cleaning, depending on the end use of the product, before a coating is applied. Soil, grease, scale, oil and other materials are usually removed by pre-cleaners, vapor degreasers, emulsion cleaners, solvent cleaners, power or mechanical washer cleaners and tumbling cleaners. Intermediate or final cleaning may precede other treatments such as alkali etches, deoxiderizers, conversion coatings, burnishing compounds or cutting compounds to produce satisfactory finishes. Such operations are quite elaborate and often costly.
There is renewed interest in electrocoating aluminum cans for food items which are processed by sterilization at about 260.degree. F. for a period of 90 minutes. Aluminum cans and aluminum alloy cans have desirable physical properties which make them attractive for use as food containers. In order to make the aluminum cans cost competitive with other food containers, can manufactures would prefer to deposit the protective organic coating on the untreated metal substrate thereby eliminating the conventional treatments such as deoxidizing, oxidizing, conversion coating, etc. as currently practiced in the art. Considerable cost savings could be effected by using untreated metal.
Substantially all commercially finished aluminum is chemically pretreated prior to the application of organic top coats. After degreasing, cleaning, and oxidizing, various conversion coatings are applied to the aluminum surface. These treatments include: (1) chrome-phosphate; (2) chrome-oxide; (3) crystalline (zinc) phosphate; and (4) amorphous (iron) phosphate. The adhesion of paint films to aluminum pretreated with these conversion coatings are rated excellent to fair in the order shown. For sanitary containers such as aluminum food and beverage cans, the prior art chrome-phosphate pretreatments provide superior adhesion of organic top coats. A particular disadvantage of this pretreatment is that rinses must be waste treated to reduce hexavalent chrome and subsequently remove trivalent chrome and fluorides. Typical procedures require a reduction step (sulfur dioxide or bisulfite) and a precipitation with lime. After filtration, the wet cake must be transported to a sanitary land fill. Because of the cost factor and environmental requirements in handling cleaners, rinses, and conversion coatings and the necessity of disposing of residuals, it is desired to provide a working process which eliminates one or more of these precoating steps. Unfortunately, when such pretreatments are eliminated the resulting coatings usually suffer from partial or total adhesion loss, especially when coated cans are used for various foods which require in-can sterilization.
Many approaches have been used to improve the adhesion or organic coatings to aluminum or its alloys. The etch treatment with a mixture of sulfuric acid and chrominum trioxide or sodium dichromate in water is quite old. The patent literature shows various means for enhancing adhesion of aluminum coatings. For example, Hoftatter, U.S. Pat. No. 4,208,223, teaches the use of epoxy functional silanols to treat aluminum surfaces as a separate step prior to painting but subsequent to a cleaning step, a deoxidizing step, and an oxidizing step. In the Hofstatter patent the cleaner can be an alkaline metal borate cleaner and the oxidation can be by chromic acid at low pH.
U.S. Pat. No. 4,243,707 (Wiggins) teaches the condensation of certain metal hydroxides or borates with certain ethylene oxide phosphate emulsifiers to provide adhesion promoters for coating metal substrates with an acrylonitrite copolymer latex.
U.S. Pat. No. 4,180,620 (Inskip) teaches the use of 0.01-0.5 weight percent magnesium, zinc, lead or calcium salts of neodecanoic acid to improve the adhesion of plasticized poly(vinyl butyral) sheets to glass, especially in laminates. These salts are applied to the sheets as a solution optionally containing anionic or non-ionic surfactants during extrusion of the sheets.
In Japanese patent application No. JP 7543799, M. Kaibu et al (C.A. 86(10)56875g) teach the immersion of an anodized aluminum alloy in an aqueous solution (0.1-50%) of a hydroxycarboxylic acid or its ammonium salt prior to electrophoretic deposition of a water-soluble acrylic resin. Coatings with improved bonding strengths result using immersion treatments of ammonium salts of tartaric and citric acid.
A similar Canadian Pat. No. CA 996497, to W. Friedman and H. G. Gerascheid, uses an aqueous solution containing 0.0005-0.5 g hydroxycarboxylic acid (citric acid) or its water-soluble salt for sealing the previously anodized aluminum surface.
B. W. Samuels, K. Sotomdek and R. Foley (Corrosion, Vol. 37, No. 2, pp. 92-93, 1971) have attempted to address the problem of corrosion of aluminum alloy 2024-T3 and have evaluated various potential inhibitors. In a controlled experiment certain treatments were applied to the metal substrate prior to a 14-day immersion test using an air saturated solution of 0.1N sodium chloride at room temperature. Test results show that sodium salts of citric acid and tartaric acid not only failed as corrosion inhibitors but indeed gave accelerated corrosion at certain concentrations. This is particularly interesting in view of the instant invention where similar organic acid salts are found to be highly effective for promoting adhesion of organic coatings to metal when the salts are prepared from multivalent metals.
Coatings, as for example epoxy and acrylic coatings, can be applied by electrocoating methods, quite often, where the substrate serves as the anode and the electrocoating tank serves as the cathode. The electrodeposition process, also referred to as electrophoresis, electrocoating, electropainting and by other names is described in some detail by M. W. Ranney in a chapter "Electrodeposition and Waterborne Coatings", Chemical Technology Review No. 97 on pages 47-91, (Noyes Data Corp., 1977). By electrodeposition is meant a process whereby aqueous compositions, including solutions, dispersions or suspensions, containing from one to 25 percent by weight of film former (resin), and usually less than 10 percent, comprises a bath in an electrocoating tank. The object to be coated and the tank are connected to opposite terminals of a high current, low voltage (up to 200 volts) DC supply. Current passes through the bath and deposits a semi-solid plastic layer on the object. The coated part is withdrawn from the bath, washed with clear water and baked in an oven to form a solid coating film on the object. These coatings can serve as prime coatings or for single coat applications. Aluminum cans, when so coated, and subjected to the required sterilization processing (90 minutes at 260.degree. F.) exhibit poor adhesion. Foods such as tomatoes, corn, and green beans adversely affect the adhesion of the epoxy-acrylic coatings. These difficulties have been overcome by the addition of selective adhesion promoters to the electrocoating formulations.